high CHT Cyl #4 - fixed

[N131MA]

Since top overhaul cylinder #4 was 30F-50F hotter than the other five jugs. In summer, it was hard to keep the #4 CHT below 400F in cruise. I went through many recommended diagnosis and fix steps: Cleaned fuel nozzles, cleaned and swapped plugs, checked JPI data for weak ignition, for induction leak and GAMI spread (.2 gal) - all no finding nor any impact.

During another detailed inspection of the baffles (looked fine) I checked the oil cooler installation: It was sealed at the bottom with high temp RTV but it had a a big hidden gap at the upper part between oil cooler and firewall. Closed this gap with high temp RTV and ... #4 temperature dropped to normal level!

Conclusion: Search for the obvious first! :-)

Edited July 18, 2016 by N131MA

[Guest]

Can you post a picture?

Clarence

[N131MA]

Hi Clarence

Absolutely! I forgot to shoot one.

I will post a picture as soon as I take off the cowling again ... or we have a look together in Oshkosh :-)

Save flight!

Peter

[THill182]

Very interesting! After my last annual I am seeing CHT's on #3 about 40F to 50F higher than the rest. All other cylinders on a very hot day, at 10k ft and LOP, 11.8 GPH FF, show <340F temps; but #3 stayed at around 380F. Not a BIG deal, but something I would like to nip-in-the-butt. Note also that #3 now goes over-peak first when leaning to the LOP side. I also suspect that it must have to do with the airflow through the cylinders. On take-off (higher angle-of-attach, and ROP), #3 runs normal or even below the others on CHT.

I'd love to see your picture also, to understand better how air flows...

PS: Forgot to mention: 2000 Ovation 2, with O3 conversion.

Thanks for any suggestions.

 

 

Edited July 20, 2016 by THill182
forgot to add type

[flyboy0681]

For us, #1 has been running 40F higher than the rest, causing me to fly with cowel flaps open and mixture very rich. It's been frustrating as hell because it means flying a Mooney without the benefits of a Mooney, slower speed and higher gph. Flying over to the Bahamas two weeks ago at 9,000 feet I glanced down and saw the JPI flashing "400".

The plane goes in for its annual on Saturday and better believe that's item #1 in bold letters.

 

[N131MA]

Hi THill

From my CHT hunting experience:

1. Figure out whether #3 has an induction leak. Sounds unlikely though. You would expect higher CHT @ ROP but lower CHT @ LOP from an induction leak. Test procedure on https://www.savvyanalysis.com/articles/in-flightdiagnostics
2. All cylinders have small inter-cylinder baffles. You need to take off the lower cowling to see them. Check whether these are properly placed and not lose. I compared #3 with #4 in order to see how correct looks like.
3. Baffle leak hunting: Check whether the baffles seal tightly, look for any holes in the firewall. You want the cooling air flowing from top to bottom of the engine. Every leak reduces this airflow. Especially important for the middle cylinders #3 and #4. This solved my problem. If you have leaks, they would likely on the side of the hotter cylinder(s). #3 is on the middle cyl on right side.
    

 

[DVA]

Hi,

In order, I’d check off the following for any higher than expected CHT condition that isn’t cured by an obvious issue (missing/misaligned air deflection inter-cylinder baffle, the rare case of a clogged injector, etc)

1. Be very sure that you are running the engine far away from the area of -5 to -50 dF ROP area as this is the range where CHTs and internal pressures will be the highest. Always be greater than at least -80 ROP.
2. Baffling and cowling - You need good air pressure on the top, with no leaks, so that the pressure differential at the bottom is enough to cause full air flow for cooling. Too many times this is ignored as a first reason for high engine temps. Just because you see baffling there doesn’t mean it’s doing its job.
3. Timing - a few degrees advanced from normal is all it takes for all cylinders to run hotter. Since most of the time you have a fairly large variation in CHT’s across the engine, timing can easily push one or two of them over the top. (whatever your ‘top’ is).
4. Check for induction leaks.
5. Make sure you have the right spark plugs installed. Often you have a choice of temperature ranges depending on the engine. For instance on the TIO-540 you can run the massive URHB32E or URHB37E plugs from Tempest. The 37’s are hotter plugs, so they will retain more heat and the CHT *can* be a bit higher as a result. I lost about 10dF on average in CHT’s by installing the colder plug of the two on my engine. Consult your A&P first.
6. Bore scope the cylinder to check for internal anomalies that may be causing higher friction.

Fly safe - have fun!
DVA

[DonMuncy]

Don't discount the possibility of an incorrect reading due to a faulty probe. From the information given, I suspect this is not the case, but always possible.

[N131MA]

Here the pictures of the oil cooler. The gap between cooler and firewall was on one upper corner. 

[kortopates]

I can't see the gap, but I assume from the picture you are referring to a gap in the rear engine baffling - probably on the center side where its close to the engine and alternator. But this isn't the firewall but rear engine baffling.

Good job though in finding and plugging the air leaks. At Savvy, we are always telling our clients to focus on the baffling after we have ruled out the other items as you have and we see symptoms like yours. Many A&Ps tell our pilots the baffling is fine, but the clients that persist and persevere as you did often make very significant progress. I am not sure what the tape is doing on the upper rear flexible baffling material - possibly covering slits? But this is a very common overlooked area. One really has to check this area carefully with the top cowling in place - putting a shop light where the alternator/oil cooler helps since if you see light coming through from the front air inlets then you know you have significant leakage. At least that how I suggest checking that area.

One also has to ensure the flexible baffling is either not so worn  and flexible or too far from the top cowl mating surface that it can easily fold over in flight allowing huge air flow losses. Newer stiff silicone baffling usually cures that if the gap is not too large.